A raindrop, a heavy one.
It hangs on a wire.
The skin of the water fights to hold it all together. But the weight pulls hard. Soon, the drop is simply too big. It shakes. Stretches. It snaps into two smaller drops.
A heavy atom works the same way. Deep inside the nucleus. Many pieces packed together. A tight grip tries to keep them close. But these pieces also fight. They repel each other. They push back hard.
In the heaviest metals, the pushing grows too strong for the tight grip. The center now. It begins to wobble. It stretches out like a pulled muscle. When it gets too long, the holding grip finally fails. The parts shove each other apart, they move away. The center breaks in two. Spontaneous fission.
Nothing hits the atom though.
It just grows too heavy.
It splits in the dark.